Ethanol-water mixtures, separation

BanaL F. A., Abu Al-Rub, F., and Shannag, M. (1999b). Modeling of dilute ethanol-water mixture separation by membrane distillation. Sep. Purif. Technol. 16, 119. 

Wines and other alcoholic beverages such as distillates represent very complex mixtures of aromatic compounds in an ethanol-water mixture. Once an extract or concentrate of the required compounds is prepared, a suitable chromatographic system must be used to allow separation and resolution of the species of interest. Many applications have been developed that use MDGC. 

Pervaporation. Pervaporation differs from the other membrane processes described so far in that the phase-state on one side of the membrane is different from that on the other side. The term pervaporation is a combination of the words permselective and evaporation. The feed to the membrane module is a mixture (e.g. ethanol-water mixture) at a pressure high enough to maintain it in the liquid phase. The liquid mixture is contacted with a dense membrane. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, thus maintaining it in the vapor phase. The permeate side is often held under vacuum conditions. Pervaporation is potentially useful when separating mixtures that form azeotropes (e.g. ethanol-water mixture). One of the ways to change the vapor-liquid equilibrium to overcome azeotropic behavior is to place a membrane between the vapor and liquid phases. Temperatures are restricted to below 100°C, and as with other liquid membrane processes, feed pretreatment and membrane cleaning are necessary. 

Figure 12.22 Distillation sequence for the separation of an ethanol-water mixture using ethylene glycol as entrainer is infeasible using single-feed column.

Al-Asheh, S., Banat, F., and Al-Lagtah, N. (2004) Separation of ethanol-water mixtures using molecular sievesand biobased adsobents. Chem. Eng. Res. Des., 82, 855. 

Zeolite/polymer mixed-matrix membranes have been investigated for liquid separations such as purification ofp-xylene [76], separation of ethanol-water mixtures [93-96] and water desalination [83]. 

The catalytic esterification of ethanol and acetic acid to ethyl acetate and water has been taken as a representative example to emphasize the potential advantages of the application of membrane technology compared with conventional distillation [48], see Fig. 13.6. From the McCabe-Thiele diagram for the separation of ethanol-water mixtures it follows that pervaporation can reach high water selectivities at the azeotropic point in contrast to the distillation process. Considering the economic evaluation of membrane-assisted esterifications compared with the conventional distillation technique, a decrease of 75% in energy input and 50% lower investment and operation costs can be calculated. The characteristics of the membrane and the module design mainly determine the investment costs of membrane processes, whereas the operational costs are influenced by the hfetime of the membranes. 

In ethanol/water mixtures addition of sodium mercaptoalkane sulfonates on vinyldiphenylphosphine proceeds smoothly at room temperature and yields a variety of tertiary phosphines such as 24. Interestingly, at the beginning of the reaction the ethanolic solution of the vinylphosphine and the aqueous solution of the educt comprise two separate phases and this is favourable for the high yields obtained (59-97%) [30]. 

Essential oils are to be distinguished from the so-called distillates which are ethanol-containing products that are obtained from plant materials by distillation with ethanol or with ethanol-water mixtures. Essence oils are defined as essential oils that separate from the aqueous phase in the distillation receiver during the distillative concentration of fruit juices (usually citrus juices). 

Water is added to the boiling solution dropwise until a small amount of product begins to separate. The mixture is then made homogeneous again by adding a few milliliters of ethanol, and the solution is cooled until the product crystallizes out. The filtered azobenzene is washed with an ice-cold 50% ethanol-water mixture yield 7-9 gm (approximately 80 %), m.p. 67°-68°C. 

Azeotropic and Partially Miscible Systems. Azeotropic mixtures are those whose vapor and liquid equilibrium compositions are identical. Their x-y lines cross or touch the diagonal. Partially miscible substances form a vapor phase of constant composition over the entire range of two-phase liquid compositions usually the horizontal portion of the x-y plot intersects the diagonal, but those of a few mixtures do not, notably those of mixtures of methylethylketone and phenol with water. Separation of azeotropic mixtures sometimes can be effected in several towers at different pressures, as illustrated by Example 13.6 for ethanol-water mixtures. Partially miscible constant boiling mixtures usually can be separated with two towers and a condensate phase separator, as done in Example 13.7 for n-butanol and water. 

Y.F. Xu, R.Y.M. Huang, Pervaporation separation of ethanol-water mixtures using ionically crosslinked blended poly(acrylic acid) (PAA)-nylon 6 membranes, J. Appl. Polym. Sci. 36 (1988) 1121— 1128. 

R.Y. Huang, C.K. Yeom, Pervaporation separation of aqueous mixtures using crosslinked poly(vinyl alcohol) (PVA). Part II. Permeation of ethanol-water mixtures, J. Membr. Sci. 51 (1990) 273-292. 

Figure 9.4 The effect of permeate pressure on the separation of ethanol/water mixtures with a polyfvinyl alcohol) membrane. The feed solution contains 20 wt% water and 80 wt% ethanol. The line drawn through the experimental data points is calculated from Equation (9.11)...

Figure 9.9 Comparison of separation of ethanol/water mixtures by distillation and by three pervaporation membranes cellulose triacetate (CTA), an anionic polyelectrolyte membrane, and GFT s poly(vinyl alcohol) (PVA) membrane [40]...

The catalytic dehydration of ethanol to ethylene in SC water may be commercially important (16). Although high quality commercial alumina catalysts exist for the vapor phase dehydration of ethanol, the commercial processes require the ethanol feedstock to be relatively free of water. Hence the ethanol must be distilled from the ethanol-water mixture which is the product of fermentation processes. By avoiding this distillation step, and securing phase separation of the ethylene product from the ethanol-water reactant, SC dehydration of ethanol could enjoy advantages over existing commercial technologies. 

Paulaitis, M.E. Gilbert, M.L. Nash, C.A. "Separation of Ethanol - Water Mixtures with Supercritical Fluids", paper presented at the 2nd World Congress of Chemical Engineering, Montreal, Canada, Oct. 5, 1981. 

As it is well known, fermentation processes generally produce aqueous solutions of ethanol, the separation of which necessitates a series of distillation processes. Distillation, however, is a very expensive operation and in addition, ethanol-water mixture forms an azeotrope which further increases the cost of purification. Therefore, novel separation methods must be investigated to make the process more feasible and economical. 

The platinum(O) compound can be prepared by a method analogous to that of Malatesta and Cariello.2 Powdered potassium hydroxide (0.350-0.400 g., 0.006 mole) is dissolved in 10 ml. of methanol contained in a large test tube (about 1 in. diameter). To this is added triethyl phosphite (2.5 g., 0.015 mole) and a small stirring bar, and then the test tube is placed in an oil bath kept at 75° by a heater-stirrer plate. When the solution in the test tube has reached the temperature of 60°, a solution of potassium tetrachloroplatinate(II), K2PtCl4 (1.24 g., 0.003 mole), in about 20 ml. of water is slowly added with stirring. Immediately or within a few minutes, colorless crystals separate. The crystals are collected on a glass frit, washed with a few milliliters of an ethanol-water mixture (50% by volume) and dried under vacuum for 4 hours at room temperature. Yields vary from 0.45 to 0.58 g. (52-67%), m.p. 114°. Anal. Calcd. for C24H6oOi2P4Pt C, 33.53 H, 7.03. Found C, 31.0 H, 7.00. 

FIG. 13-52 Azeotropic distillation tower for distillation of an ethanol-water mixture using benzene as a mass separating agent. [Ajier Prokopakis and Seider (op. cit.).]... 

Alginate hydrogel membranes, crosslinked with EIX2, can be used in the pervaporation separation of ethanol-water mixtures. ... 

The high solubility and high stability of substituted polyacetylenes are the two most important properties which are not seen with polyacetylene. Consequently, stable membranes can be easily obtained by casting solutions of substituted polyacetylenes. This will greatly facilitate their application. Here, we refer to several functions of substituted polyacetylenes, which might be applied to oxygen enrichment of air, separation of ethanol-water mixtures, and so on. 

The mass flow is effected by keeping the downstream side of the membrane at reduced pressure. The performance of membranes for the pervaporation of ethanol-water mixtures is evaluated by the separation factor a H and the specific permeation rate R. is defined as follows ... 

Another application of substituted polyacetylenes is in the pervaporation of the ethanol-water mixture. Pervaporation is a method for separating liquid mixtures by evacuating the downstream side of the separation membrane (29). The performance of a membrane is evaluated by the separation factor (a) and the specific permeation rate (R). 

Similarly, Sano et al. [1994] added colloidal silica to a stirred solution of tetrapropylammonium bromide and sodium hydroxide to synthesize a hydrogel on a stainless steel or alumina support with a mean pore diameter of 0.5 to 2 pm. The composite membrane is then dried and heat treated at 500 C for 20 hours to remove the organic amine occluded in the zeolite framework. The silicalite membranes thus obtained are claimed to be free of cracks and pores between grains, thus making the membranes suitable for more demanding applications such as separation of ethanol/water mixtures where the compound molecules are both small. The step of calcination is critical for synthesizing membranes with a high permselectivity. 

Sano T., Yangishita H., Kiyozumi K., Mizukami F. and Haraya K., Separation of ethanol-water mixture by silicalite membrane on pervaporation, 7. Membr. ScL 95 221 (1994). 

A good example of separation on the basis of affinity is the separation of alcohol/ water mixtures using a hydrophobic, silicalite membrane. Pervaporation of an ethanol/ water mixture through such a membrane resulted the removal of the alcohol from the mixture [16]. The separation selectivities achieved are between 10 and 60, depending on temperature and the alcohol content in the feed. In this way azeotropes can be broken. The reason for this is that the principle of separation, namely, differences in adsorptive behavior, is different from separation based on vapor pressure differences, used in distillation. Another example of such a separation is the pervaporation of an acetic acid/water mixture through a silicalite membrane, resulting in the removal of acetic acid [17]. 

Lee KH, Kim HK, and Rhim JW. Pervaporation separation of binary organic-aqueous Uquids mixtures using crosslinked PVA membranes III ethanol-water mixture. J. App. Poly. Sci. 1995 58(10) 1707-1712. 

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